1. Field of the Invention
The present invention is related to a driving circuit of a piezo-ceramic transformer and a dimming control method thereof, more particularly, to a driving circuit of a piezo-ceramic transformer capable of controlling an input voltage and a dimming control method thereof without a DC--DC converter.
2. Description of the Prior Art
Technology for making a relatively thin and small notebook computer with a LCD (Liquid Crystal Display) is being developed. It is therefore desirable to develop a DC-AC inverter that is relatively small and has a relatively high efficiency to use as an LCD backlight.
However, a magnetic transformer used with a conventional DC-AC inverter for a LCD backlight has limits for making it small in size and obtaining a high efficiency.
Although an inverter using a PCT (Piezo-Ceramic Transformer) for improving the above conventional problems has been studied for several years, it also has several problems in that the driving circuit is very complicated and another DC--DC converter is needed in order to constantly maintain an applied voltage to the PCT driving circuit because in the application circuit, an input voltage has a wide variable range.
That is, general types of the PCT being used practically are as follows:
1. a buck or boost DC--DC converter+a push-pull type inverter, PA1 2. a DC--DC converter+a single ended quasi resonant type inverter, PA1 3. a DC--DC converter+a half bridge type inverter, PA1 4. a DC--DC converter+a flyback single ended quasi resonant type inverter.
Since the above types are all operated by two stage inverter, a switching device and a control circuit in every singe stage inverter must be used, thereby making the circuit very complicated.
FIG. 7 is a circuit diagram of a conventional a half bridge type inverter with a piezo-ceramic transformer.
As shown in FIG. 7, when an input voltage is changed from Vmin to Vmax, a constant voltage is made at the point A' of the circuit by the DC--DC converter and provided to transistor Q1 and Q2 for driving a PCT, as a switching device. In the circuit, the duty ratio of the transistor Q1 and Q2 are continuously controlled so that a constant voltage is applied to point C' at the input terminal of the PCT.
In the case of resonant type inverter using the magnetic transformer, a single stage inverter is adopted to drive the CCFL lamp. A wide range input line regulation may be controlled using frequency modulation. But the method of controlling frequency modulation has a critical problem in that the brightness efficiency of the lamp fluctuates by varying the driving frequency of the magnetic transformer or the PCT. As a result, even thought the electrical efficiency of the lamp is maintained at a constant value, the brightness efficiency of the lamp rapidly fluctuates with the variation of driving frequency. Because the Q(Quality factor) value of the PCT is much higher than the magnetic transformer, the frequency modulation method to drive the PCT results in higher brightness fluctuation than to drive the magnetic transformer.